Chemical mining of copper porphyry ores

ABSTRACT

Copper porphyry ores, especially those too deeply buried for conventional open pit mining, are mined in place by an in situ leaching technique using as a leaching medium a mixture of dilute sulfuric acid, oxygen and an alkali metal or ammonium chloride salt. The chloride ion speeds dissolution of copper minerals, especially chalcopyrite, and the alkali metal or ammonium ion reacts with iron and sulfate in the leaching medium to deposit iron in the form of crystalline jarosites. Precipitation of iron within the ore body as a jarosite maintains the permeability of the ore body to the leaching medium thus increasing both the rate and the total recovery of copper as well as depleting the leach solution of unwanted iron.

United States Patent Heinen et al.

CHEMICAL MINING OF COPPER PORPHYRY ORES Inventors: Harold J. Heinen,Reno; Thomas G. Carnahan, Sparks; Judith A. Eisele, Verdi, all of Nev.

The United States of America as represented by the Secretary of theInterior, Washington, DC.

Filed: Feb. 7, 1974 Appl. No.: 440,432

Assignee:

References Cited UNITED STATES PATENTS Johnson 423/150 X 3/1974 Weston423/150 X June 17, 1975 3,823,981 7/1974 Lewis 299/4 PrimaryExaminer-Frank L, Abbott Assistant Examiner-William F. Pate, Ill

Attorney, Agent, or Firm-Roland H. Shubert; Donald R. Fraser 57 ABSTRACTCopper porphyry ores, especially those too deeply buried forconventional open pit mining, are mined in place by an in situ leachingtechnique using as a leaching medium a mixture of dilute sulfuric acid,oxygen and an alkali metal or ammonium chloride salt. The chloride ionspeeds dissolution of copper minerals, especially chalcopyrite, and thealkali metal or ammonium ion reacts with iron and sulfate in theleaching medium to deposit iron in the form of crystalline jarosites.Precipitation of iron within the ore body as a jarosite maintains thepermeability of the ore body to the leaching medium thus increasing boththe rate and the total recovery of copper as well as depleting the leachsolution of unwanted iron.

8 Claims, No Drawings CHEMICAL MINING OF COPPER PORPHYRY ORES mineralsoccur as the discrete grains and veinlets throughout a large volume ofrock which commonly is porphyry. The deposits are. typically largetonnage but low grade, having an average copper concentration of lessthan about 1 percent. Copper minerals found in these deposits usuallyare sulfides and most commonly are chalcopyrite. When such a deposit isof sufficiently high grade, and either outcrops on the surface or issufficiently close to the surface, then the ore is mined by open pitmethods and the copper minerals are separated from the gangueconstituents by techniques such as flotation. Deeply buried or very lowgrade copper porphyry deposits cannot be economically exploited at thistime.

It has been proposed to extract the cooper from deeply buried porphyrydeposits by in situ leaching techniques. In situ leaching is awell-known technique which has long been practiced; its origins can betraced as far back as the th century. Successful leaching operations atthis time are restricted to those copper ore bodies containing copper inthe oxide form and having a low percentage of calcareous minerals.Copper oxides such as azurite and malachite are readily soluble indilute sulfuric acid. If high concentrations of calcareous minerals suchas calcite and dolomite are present, then acid usage becomes so high asto make the process uneconomical.

Copper sulfide minerals in general are but very slowly and sparinglydissolved by dilute sulfuric acid. Of the copper sulfide minerals,chalcopyrite is probably the least soluble yet it is the most commoncopper sulfide. It is known that the use of an over-pressure of oxy genin conjunction with an aqueous sulfuric acid leach medium speeds thedissolution of the copper sulfide minerals. A description of such atechnique was reported by Vizolyi et al in an article entitled Copperand Elemental Sulfur from Chalcopyrite by Pressure Leaching which waspublished in the Journal of Metals, volume 19, No. 11 (1967), pages52-59. Vizolyi found that finely ground chalcopyrite concentrates couldbe dissolved in an autoclave using an oxygen partial pressure in therange of about -500 pounds per square inch at a temperature in the rangeof about 200 to 300F. Most of the sulfide sulfur was oxidized toelemental sulfur and most of the iron contained in the chalcopyrite waseventually hydrolyzed during the leaching to ferric hydroxide. Otherresearchers have investigated the effect of sodium chloride additions tosulfuric acid leach solutions in the extraction of copper fromchalcopyrite ores. This work was reported by Dutrizac et al. in articleentitled The Effect of Sodium Chloride on the Dissolution ofChalcopyrite Under Simulated Dump Leaching Conditions and appearing inMetallurgical Transactions, volume 2, (1971), pages 2310-2312. Theyfound that sodium chloride accelerates the dissolution of chalcopyriteonly at temperatures above about 50C and had a detrimental effect on therate of chalcopyrite dissolution at lower temperatures. During leachingis a technique used to extract copper from a low grade waste materialproduced during the large scale open-pit mining of copper ore deposits.Leach solution is sprayed onto the top of the dump, percolates throughthe dump, and is collected from a lower level. Temperature of theleaching medium is dependent strongly upon ambient air temperature andseldoms exceeds about 35C..l-Ien'ce, the use of sodium chloride as anadditive to sulfuric acid leach solutions isof no benfit and is probablydetrimental to all opera- I tions carried out at ambient temperatures.

One 'of'the most troublesome problems encountered in any copper leachingoperation, whether it is a surface leaching processor whether it is anin situ leaching operation is theprecipitation of basic iron salts inpipe lines, on the surface of dumps, or within the dump or ore bodyitself. Iron hydroxide precipitates from solution when the pH of thesolution is higher than about 3.0. Iron salt precipitation within dumpsor within an ore body is extremely difficult to control since leachsolution concentrations and pH are subject to wide local variation.Basic iron salts precipitation results in the formation of imperviousgelatinous layers which prevent movement of leach solutions within thedump or ore body. Hence, copper bearing minerals which are coated withprecipitated iron hydroxide have no contact with leach solutions andtheir copperv content cannot be recovered no matter how long theleaching is continued.

SUMMARY OF THE INVENTION We have found that chalcopyrite .can be readilyleached from copper porphyry ores while avoiding the detrimental effectsof ferric hydroxide precipitation by using as a leaching medium a dilutesulfuric acid solution to which is added chloride ion and cation chosenfrom the group consisting of sodium, potassium and ammonium. Theleaching step must be carried out at temperatures above about 50C andoxygen must be present. Our process is especially well suited to thesitu leaching of deeply buried porphyry copper deposits which have beenshattered by either conventional or atomic explosives. Essentially allof the co-dissolved ferric iron is precipitated in situ as crystallinejarosites which do not impede flow of the leach solutions. Pregnantleach solutions recovered in our process display very low ironconcentration. In addition, most of the sulfur remains deposited withinthe ore body in the elemental form. A paper describing our invention andentitled Simulated In Situ Leaching of Copper from A Porphyry Ore waspublished as Bureau of Mines Technical Progress Report 69, dated May,1973.

Hence, it is an object of our invention to extract copper from porphyryores.

A specific object of our invention is to provide an in situ leachingprocess to dissolve chalcopyrite and to deposit iron dissolved by theleach solution as a crystal- 5 line jarosite within the ore body.

chloride salts to a dilute sulfuric acid leach solution in combinationwith gaseous oxygen speeds the dissolution of copper-iron sulfides suchas chalcopyrite and precipitates the iron from the leach solution as acrystalline jarosite. Chloride salts suitable forum in our processinclude sodium chloride, potassium chloride, and ammonium chloride. Ofthese salts, sodium chloride is preferred on the basis of its price andavailability. In order to achieve a full benefit from our leachingprocess, certain conditions must be met. Oxygen must be present with theleach solution. Leaching tempera-' ture must be above about 60C andpreferably in the range of 60 to,l80C. Presence of chloride ion inassociation with sulfuric acid approximately doubles the rate ofdissolution of copper-iron sulfides such as chalcopyrite provided thatthe temperature of the leach solution is above about 50C; there islittle if any enhancement of dissolution rate attributable to thechloride ion at ambient temperature.

Another condition that must be met in order to successfully practice ourinvention is to allow or cause the pH to rise to 0.5 or greater duringthe leaching process. Formation of jarosites is both pH and temperaturedependent, ocurring only at a pH of about 0.5 to 3.0 and at atemperature of about 60 to 70C or greater. At a pH higher than about3.0, iron precipitates as the hydroxide rather than as a jarosite.

Jarosites are crystalline compounds having the general formula MFe(SO.,) (OH) where M designates a cation chosen from the group consistingof sodium, potassium and ammonium ions. All three of these compoundsoccur in nature. They typically occur as sandlike crystals havingayellow or yellow-brown color. The crystals will spontaneously form inaqueous solutions containing ferric, sulfate and alkali metal orammonium ions provided that the solution is acidic but has a pH of 0.5to 3.0 and provided that the temperature is above about 60 to 70C.Experimental work indicates that essentially complete removal of ironfrom leach solutions can be accomplished at a temperature of 120 to 150Cby precipitation as sodium jarosite. lron removal is somewhat lesscomplete at lower temperatures.

Several important advantages accrue to our process by precipitation ofiron as a jarosite. Because the jarosites are crystalline and sand-like,their precipitation within the ore does not adversely affect theporosity or permeability of the ore to leach solutions. Thus, the leachsolution may permeate through the precipitated jarosite grains tocontact sulfide mineralization in the ore allowing improved recovery ofcopper and pregnant solution. In contrast, when iron precipitates as thebasic sulfate or as a hydrated oxide, it deposits as a gelatinous-typematerial forming layers through which leach solutions cannot pass.Another advantage lies in the removal of unwanted iron from solution.Iron, particularly ferric iron, increases the complexity and expense ofrecovering copper from the pregnant leach solution.

The important variables in our process are solution composition,temperature and oxygen pressure. Solution composition has a pronouncedeffect on the copper extraction as well as on the precipitation ofcodissolved ferric iron as crystalline jarosites. Sulfuric acidconcentration in the leach solution must be sufficient to maintain thepH below about 3 in order to dissolve the copper sulfide minerals. Muchhigher acid concentrations are preferred, and the pH of the entering, orlean,;leach solution may be 1 or below. Acid may be added directly tothe leach solution, or if pyrite is present in the deposit, sulfuricacid will be generated by oxidation of the pyrite during the leachcycle. Concentration of sodium, potassium, or ammonium ion in the leachsolution must at least be sufficient to provide stoichiometric amountsof those ions for the precipitation of the co-dissolved ferric iron as ajarosite. It is preferred that the chloride salts be present in excessover that theoretically required as this excess accelerates both thedissolution of the copper-iron sulfides and formation of jarosites.

' As has been previously stated, the temperature of our leaching processmust be above about 60C in order to speed dissolution of the copperironsulfides and to allow precipitation of jarosites. Temperature also hasan effect upon the conversion of sulfide sulfur to elemental sulfur. Itis generally advantageous to seek the maximum conversion of sulfidesulfur to elemental sulfur as that alleviates a problem of wastedisposal and treatment. Elemental sulfur formed remains to a largeextent within the interstices of the leached ore. Maximum conversion ofsulfide sulfur to elemental sulfur in our process occurs at temperatureof about 100C. At this temperature, some of the sulfide sulfur isconverted to the elemental form. Higher or lower tempera tures tend todecrease the formation of elemental sulfur.

Free oxygen in association with our leach solution performs the functionof oxidizing sulfide sulfur to the elemental form and in some cases tothe sulfate form. One of the chemical reactions taking place duringleaching is the oxidation of pyrite to produce sulfuric acid and ferroussulfate which in turn is further oxidized to ferric sulfate. The ferricsulfate-sulfuric acid solution, in association with oxygen, thendissolves chalcopyrite and other copper sulfide minerals present in theporphyry ore. Oxygen over pressure has a definite effect on the rate ofoxidation and dissolution of chalcopyrite when using the sulfuricacid-chloride salt leach solutions of our process. As a general rule,the higher the pressure the faster the chalcopyrite dissolution. Theincrease in dissolution rate of chalcopyrite caused by an increasedoxygen over pressure is more pronounced at lower temperatures than athigh temperatures. Enough oxygen must be provided to oxidize sulfidesulfur contained in the ore to stable forms which may be elementalsulfur or sulfate. We prefer to operate our porcess with oxygen overpressures of at least 25 psi. Little increase in dissolution rate ofchalcopyrite was observed at oxygen over pressures greater than about200 psi. However, such higher pressures may be used. Oxygen may besupplied as air, oxygen enriched air, or pure oxygen.

Because of the limitations of temperature and oxygen over pressure, ourprocess is not feasible for use in conventional surface heap leaching.While our process can be practiced in an autoclave-type reactor, suchuse would seldom be economically practical because of the low value ofcopper porphyry ores. Our process is, however, admirably suited for usein the in situ leaching of copper porphyry deposits; particularly deeplyburied porphyry deposits which cannot be economically worked byconventional methods.

Porphyry ore deposits seldom if ever have sufficient naturalpermeability to either liquids or gases to allow a successful leachingoperation to proceed. Hence, an extensive fracture system must becreated within the ore body before leaching. Fracturing has beenaccomplished by the use of conventional chemical explosives as well asby hydraulic means. It has also been proposed to fracture deep orebodies by means of nuclear blasting. This technique has been shown to bepracticable in the test known as the Piledriver event which wasconducted in granite. Such a nuclear blast creates a verticallyoriented, cigar-shaped, fractured chimney caused by overlying rockscollapsing into the spherical shaped blast zone of the nuclearexplosive. A substantial proportion of the rock sizes within thefracture chimney would have sizes of minus 12 inches in diameter andwould display many more micro fractures than rocks shattered byconventional blasting because of the intense shock waves created by thenuclear blast.

Initial temperatures within the shattered ore zone are dependent uponthe depth of the ore body, the local geothermalgradient and the methodof blasting. Typical initial temperatures within the fractured ore bodywould be in the neighborhood of 60C. Oxidation of the pyrite andchalcopyrite minerals with its attendant release of heat would furtherraise temperatures within the ore body to an expected level of about 150to 200C. Hydrostatic pressures within the fractured ore body are afunction of depth of the fractured zone and are on the order of 1,000psig at 2,000 foot depth. Leaching of the fractured ore body isaccomplished by introducing oxygen and barren leach solution throughwells from the surface to the base of the fractured zone. Other wellscommunicating between the surface and the top of the fractured zonerecover pregnant leach solution and vent gas accumulating at the top ofthe zone. The pregnant leach solution is then processed at the surfaceto remove its dissolved copper and is recycled back to the leachingzone. Recovery of copper from the pregnant leach solution may beaccomplished by such conventional techniques as cementation,precipitation, and electrowinning. Of these approaches, electrowinningis preferred because the pregnant leach solution, being low in iron, isa desirable feed for that operation. Electrowinning also has theadvantage of regenerating sulfuric acid for recycle to the leachingzone.

The following examples serve to more fully illustrate the effect ofprocess variables on the practice of our invention.

EXAMPLE 1 Samples of porphyry copper ore were obtained from a Nevadacopper mine. The principle copper mineral was chalcopyrite with somebornite. Little if any pyrite was present. About one-third of the ironvalues were combined with copper as chalcopyrite and about two thirds ofthe iron was present as complex silicates. The principle ferruginousgangue material was epidote with smaller amounts of chlorite andbiotite. Only trace amounts of calcite were present. Copper, iron, andsulfur contents of the ores were 1.1, 3.4, and 1.0 percent respectively.A series of pressure leaching tests were conducted on samples of theporphyry copper ore crushed to three-eights of an inch diameter usingsulfuric acid solution with and without additions of sodium chloride.The leaching was conducted under 200 psi of oxygen over pressure for 64hours. Temperatures in successive tests were varied from 100 to 300C.Results obtained are shown in Table 1.

As may be seen from the table,'the effect of sodium chloride additionsto sulfuric acid are most pronounced at the lower temperatures. Thepercentages of copper and iron extracted represent the amount of theoriginal metal content of the ore which was recovered in the leachsolution. Iron extractions in the last three tests are very low becauseof the precipitation of extracted iron as sodium jarosite within the oreparticles.

EXAMPLE 2 Additional tests were carried out to delineate the ef fect ofleach solution composition upon the dissolution of chalcopyritecontained in the porphyry ore used in Example 1. Crushed ore chargeswere digested at C under an oxygen atmosphere for 336 hours at varyingconcentrations of sulfuric acid and sodium chloride.

Results obtained are set out in table 2.

Table 2 Composition of Extractions leach solution %Cu %Fe lMH SO+0.l6MNaCl 18 21 l M H 80 0.8 M NaCl 43 14 1 M 11,80 3.3 M NaCl 81 2 4 M11,80 0.16 M NaCl 60 58 4 M H 50 0.8 M NaCl 80 69 4 M H 80, 3.3 M NaCl91 69 The leach solutions made up with 0.16 M sodium chloridetheoretically contained just enough sodium ions to react with 80% of thetotal iron to form the jamsite. The data shown that increasing thesodium chloride concentration increases the rate of chalcopyritedissolution at both acid concentrations used in the tests. Ironextraction was much greater in the 4 M acid solutions than in the l Macid solutions. This result is attributed to the fact that the pH wastoo low for jarosite formation in the stronger acid solutions. Resultsof the test using 1 M acid solutions show a definite trend of decreasingiron in solution as copper extraction increased. This trend is to beexpected since the pH of the leach solution rises as copper extractionincreases thus favoring the formation of jarosites.

EXAMPLE 3 A series of pressure leaching tests were conducted to comparethe effect of sodium chloride with that of sodium sulfate as additivesto a sulfuric acid leach solution. A copper porphyry ore having acomposition of that used in Example 1 was crushed to a inch particlesize and was leached at C under 200 psi of oxygen for 64 hours. Sodiumcompounds were added in such amount as to provide an equal concentrationof sodium ion in the leach solution. Results obtained are set out in thefollowing table.

Table 3 Leach solution composition Extractions Cu %Fe l M H 30 27 24 l MH 80 3.3 M NaCl 83 4 l M H 80 1.7 M Na SO 25 19 Addition of sodiumsulfate to sulfuric acid had little if any effect on the extraction ofcopper and iron from the'porphyry ore as compared to sulfuric acid usedalone. Theoretically, sodium sulfate furnished sodium ions for the insitu precipitation of co-dissolved iron as jarosites. No such result wasobserved in these tests but that is attributed to the acidity of theleach solution rather than to the lack of sodium ions.

EXAMPLE 4 Tests were made to determine the effect of oxygen overpressure on the dissolution of chalcopyrite from the copper porphyry oreof Example 1. Several charges of ore were treated with a l to 1 weightratio of a 1.0 M H 80 1.6 M NaCl solution at varying temperatures andoxygen pressures for 48 hours. Results are shown in the following table.

Table 4 Temperature, 25 psi 200 psi 0 C extraction extraction %Cu %Fe%Cu %Fe The codissolvcd ferric iron was precipitated as a natrojarositc.

EXAMPLE tion of sodium chloride. Sulfuric acid concentration in test No.19 was also 1.9 molar. Results obtained are shown in the followingtable.

Table 5 Test Temperature, Cu extraction. 8 converted to 5.

No. "C pct. pct.

v l 8 I00 62 84 These data show that the optimum conversion of sulfidesulfur to elemental sulfur occurs at a temperature EXAMPLE 6 A two-inchdiameter specimen of the copperporphyry ore of Example 1 was leachedwith a sulfuric acid-sodium chloride solution for 14 days at 120C under200 psi oxygen over pressure. Acid concentration of the leach solutionwas 1.9 molar and sodium chloride concentration was 3.3 molar. Visualinspection of the leached rock showed that the continuous sulfideveinlets had been dissolved. Finely disseminated sulfides withinone-sixths inch from the surface or from a fracture were also leached.Water freely percolated through the leached specimen.

Visual inspection also showed the presence of globules of elementalsulfur. Presence of natrojarosite deposited within the leached specimenwas confirmed by X-ray diffraction analyses. Chemical analyses wereperformed both on the leached specimen and upon the pregnant leachsolution. These analyses indicated a conversion of sulfide sulfur toelemental sulfur and a copper extraction of 53%. The leach solutioncontained 20.0 grams of copper per liter and only 24 ppm of iron. Theseexamples are for the purpose of more fully describing and illustratingour invention. Minor variations and changes will be obvious to thoseskilled in the art.

We claim:

1. A process for extracting copper from a porphyry ore containingchalcopyrite which comprises:

contacting said ore with a leach solution comprising an aqueous mixtureof sulfuric acid and a salt chosen from the group consisting of sodiumchloride, potassium chloride and ammonium chloride in the presence offree oxygen at a temperature above about 60C for a time sufficient todissolve chalcopyrite contained in said ore and to attain a pH of saidleach solution of above about 0.5 but below about 3.0 therebyprecipitating iron dissolved by said leach solution within the ore as agranular, crystalline jarosite having the formula MFe (SO (Ol-i) whereinM is chosen from the group consisting of sodium, potassium and ammoniumions;

separating spent, copper-containing leach solution from the ore, and

recovering copper from the leach solution.

2. The process of claim 1 wherein said porphyry ore is a buried ore bodyand wherein a zone in said ore body is first fractured and the fracturedore is thereafter leached in place.

3. The process of claim 2 wherein leaching of the fracutred ore body isaccomplished by introducing barren leach solution and oxygen throughwells communicating between the surface and the base of the fracturedore zone and wherein pregnant leach solution and gases are removedthrough wells communicating between the surface and the top of thefractured ore zone.

4. The process of claim 3 wherein the salt is sodium chloride.

5. The process of claim 4 wherein the concentration of sodium chloridein the leach solution is substantially in excess of that required tofurnish sodium ions for the electrolyte from the electrowinning step isrecycled to the fractured ore zone.

8. The process of claim 5 wherein the leaching step is carried out attemperatures of about C thereby maximizing the conversion of sulfidesulfur to elemental sulfur within the fractured ore zone.

1. A PROCESS FOR EXTRACTING COPPER FROM A PORPHYRY ORE CONTAININGCHALCOPYRITE WHICH COMPRISES: CONTACTING SAID ORE WITH A LEACH SOLUTIONCOMPRISING AN AQUEOUS MIXTURE OF SULFURIC ACID AND A SALT CHOSEN FROMTHE GROUP CONSISTING OF SODIUM CHLORIDE, POTASSIUM CHLORIDE AND AMMONIUMCHLORIDE IN THE PRESENCE OF FREE OXYGEN AT A TEMPERATURE ABOVE ABOUT60*C FOR A TIME SUFFICIENT TO DISSOLVE CHALCOPYRITE CONTAINED IN SAIDORE AND TO ATTAIN A PH OF SAID LEACH SOLUTION OF ABOVE ABOUT 0.5 BUTBELOW ABOUT 3.0 THEREBY PRECIPITATING IRON DISSOLVED BY SAID LEACHSOLUTION WITHIN THE ORE AS A GRANULAR, CRYSTALLINE JAROSITE HAVING THEFORMULA MF3(SO4)2(OH)6 WHEREIN M IS CHOSEN FROM THE GROUP CONSISTING OFSODIUM, POTASSIUM AND AMMONIUM IONS; SEPARATING SPENT, COPPER-CONTAININGLEACH SOLUTION FROM THE ORE, AND RECOVERING COPPER FROM THE LEACHSOLUTION.
 2. The process of claim 1 wherein said porphyry ore is aburied ore body and wherein a zone in said ore body is first fracturedand the fractured ore is thereafter leached in place.
 3. The process ofclaim 2 wherein leaching of the fracutred ore body is accomplished byintroducing barren leach solution and oxygen through wells communicatingbetween the surface and the base of the fractured ore zone and whereinpregnant leach solution and gases are removed through wellscommunicating between thE surface and the top of the fractured ore zone.4. The process of claim 3 wherein the salt is sodium chloride.
 5. Theprocess of claim 4 wherein the concentration of sodium chloride in theleach solution is substantially in excess of that required to furnishsodium ions for the precipitation of all ferric iron dissolved by thesolution as natrojarosite.
 6. The process of claim 5 wherein the oxygenover pressure within the leaching zone is in excess of 25 psi.
 7. Theprocess of claim 5 wherein copper is recovered from the pregnant leachsolution by electrowinning and wherein barren leach solution comprisingelectrolyte from the electrowinning step is recycled to the fracturedore zone.
 8. The process of claim 5 wherein the leaching step is carriedout at temperatures of about 100*C thereby maximizing the conversion ofsulfide sulfur to elemental sulfur within the fractured ore zone.